fix empty feed parsing

doc/changelog.txt

@@ -5,6 +5,7 @@
 - (fix) handle opml files not following the spec (thanks to @huangnauh for the report)
 - (fix) pagination in unread/starred feeds (thanks to @Farow for the report)
 - (fix) handling feeds with non-utf8 encodings (thanks to @fserb for the report)
+- (fix) errors caused by empty feeds (thanks to @decke)
 - (fix) ui tweaks (thanks to @Farow)
 
 # v2.0 (2021-04-18)

src/parser/feed.go

@@ -18,6 +18,11 @@ type processor func(r io.Reader) (*Feed, error)
 func sniff(lookup string) (string, processor) {
 	lookup = strings.TrimSpace(lookup)
 	lookup = strings.TrimLeft(lookup, "\x00\xEF\xBB\xBF\xFE\xFF")
+
+	if len(lookup) < 0 {
+		return "", nil
+	}
+
 	switch lookup[0] {
 	case '<':
 		decoder := xmlDecoder(strings.NewReader(lookup))